Command pod blows up before heat shield uses all ablator

[Wizard Kerbal]

Interplanetary return at the regular 4,500 m/s, Nuclear tug separation, lander can blows up. The heat shield flies away, still with half ablator. The same re-entry setup worked on my Gilly mission, just with a bit slower re-entry at 4,200 m/s. Wat?

And the worst part was it was after an Eve return. All that for a kaboom in the upper atmosphere.

Edit: Ended up turning re-entry heating to 90%. Weird glitch after “cheating” to a safe splash down. My nuclear tug survived re-entry, and splashed down successfully. 
I tried a steeper approach, setting my apoapsis to 30 km. Instantaneous death, but 30 km and deceleration burn worked, without cheats.

Edited March 24, 2021 by Wizard Kerbal

[paul_c]

Someone more knowledgeable will doubtless correct me, but I believe KSP calculates heat loads in two ways: 1) surface temperature. The ablator normally takes the brunt of this. 2) heat transfer by conduction from the surface to the innards; and from connected components. I don't know the full details of how this is evaluated.

So it sounds like the heatshield is doing its job but all the while, the command module is getting "cooked".

Also, there's a theory that a shallow entry with a long drawn out slowdown is worse, something to do with the overall heating being to the power of 4 of the speed while the instantaneous heating is to the square of the speed. So a more efficient entry is to drop straight down, except it will impose greater acceleration forces and damage components/kill Kerbals.

I believe there's an info page which will show the numbers of the various temperatures of the components too, which will illustrate what is happening too - don't know the button to press though.

[jimmymcgoochie]

Lander can? As in the grey octagonal Mk1 lander can, or the white circular/cuboidal Mk2 lander can?

No wonder it exploded- those things are not meant for re-entry and have really poor heat tolerance; the thermal conduction from the heat shield (attached directly to the underside of the lander can perhaps?) would be enough to overheat the lander can, assuming the heat shield was large enough to completely shield the can from airflow.

You’d be surprised how much of a difference even a relatively small increase in speed makes to re-entry heating, so slowing down as much as possible is recommended. A steeper descent will mean you reach the thick part of the atmosphere sooner and slow down more rapidly, meaning a sharper thermal peak but less overall heating because you slow down quickly, whereas skimming through the upper atmosphere means you get most of the heating but over a much longer time period as you don’t slow down as quickly, which is where parts with low core heat tolerances (like lander cans) tend to go boom.

[OHara]

The Mk1 lander can sticks out beyond the 1.25-meter heat shield. KSP applies shock heating to the exposed skin of the lander can, the bits sticking out beyond the diameter of the heat shield.

If your lander is a different part, maybe a little tipping of the vehicle exposed the lander to shock heating.  The symptom sounds very much like an exposed part behind the heat shield.

You can use the debug popup windows to see KSP simulation of heat.  The heat shield acts as if it is very well insulated, letting only its skin get hot and transmitting very little to connected parts.

(Ablator in KSP does not do much.  KSP does not simulate any larger bow shock due to evaporating ablator.  Consumption of ablator removes so little heat from the heat-shield's skin, that ablator rarely helps.)

Edited March 24, 2021 by OHara
forgot to mention heat shields' apparent insulation

[Wizard Kerbal]

6 hours ago, jimmymcgoochie said:

No wonder it exploded- those things are not meant for re-entry and have really poor heat tolerance; the thermal conduction from the heat shield (attached directly to the underside of the lander can perhaps?) would be enough to overheat the lander can, assuming the heat shield was large enough to completely shield the can from airflow.

I was using the Mk 2, and the lander cans are lighter than the Mercury and Apollo pod, So I figured I would use it just to remove a little extra mass. Must’ve been the wrong decision.

[king of nowhere]

by the way, 4500 m/s seems a bit too much for an Eve return. I would associate it with a Jool return